The present invention relates to a device and a method for a driving section in connection with the manufacturing of paper or paperboard as a fibrous web which drying section comprises a drying drum which drying drum is rotatable about a first axis of rotation and is arranged such that the fibrous web is brought to run in contact with the surface of the drying drum, at least about a part of its circumference and to be dried by the same.
At the manufacturing of paper or paperboard in a paper or paperboard machine a fibrous web, having a relatively low dry substance content, is first formed on a wire. Thereafter, the fibrous web is dewatered in a press section, in order to subsequently proceed to a drying section for additional enhancing of the dry substance. The limit in dry substance, between the press section and the drying section, could be said to be about 40-50% dry substance. The fibrous web leaves the drying section at a dry substance of about 90-95%, in order to thereafter be further treated in the form of glossing, coating, lamination etc.
In the drying section, the fibrous web runs in abutment against a number of drying drums. Typically, the drying section comprises about 20-40 drying drums. Thereby, the fibrous web runs up and down, in ziezag, between these drying drums, abutting the surfaces of the drying drums at about half of their respective circumference. An endless drying wire is arranged to support the fibrous web along a part of the section against which the fibrous web abuts the surface of the drying drum. Usually, the drying drums are heated, normally by steam, and the fibrous web is dried against each drying drum by heat transfer to the fibrous web, or to its content of moist.
In connection with conventional drying sections, there is a problem in that the fibrous web tends to contract, i.e to shrink, along its longitudinal edges, in the cross direction (CD) for the paper or paperboard machine, whereby it will instead increase in thickness (z) alone the longitudinal edges. This leads to an increase in basis weight, thickness and surface coarseness in the area of the longitudinal edges, in relation to the case if no cross directional shrinking had occurred. As a result of this, the longitudinal edges may exhibit flaws in the edge and an uneven paper or paperboard quality.
In its turn, this will cause problems in subsequent post treatment. In connection with blade coating e.g., disruptions will arise due to the uneven edge quality and moreover the coating result will be uneven. Also, the blade which is used for the blade coating will be more rapidly worn at the edges as compared to its more central parts, as seen in CD, which causes it to be exchanged unreasonably often. In particular, this is a problem in connection with the use of a soft tip blade.
The increase in basis weight, thickness and surface coarseness may also impair other post treatment of the paper. This may e.i. be the case for glossing or calendering.
The present invention aims at diminishing or eliminating the above problems, by providing a device and method for a drying section, by use of which even longitudinal edges may be attained in the fibrous web. When the invention is used, the fibrous web will accordingly exhibit essentially the same surface coarseness and thickness along the area of its longitudinal edges as in the more central part of the fibrous web, as seen in CD. Hereby, a subsequent coating is facilitated, which coating will have a more even result and will wear the coating equipment less. The wearing may especially be decreased in case of a soft tip being used in blade coating.
Another advantage which is attained by levelling of the edge is that impurities (e.g. fibrous clumps, shives or alien objects) also are pressed into the web. Thereby, their possible effect in causing disruptions in subsequent treatment. e.g. coating, glossing or super-calendering, is decreased.
According to the invention, this is achieved by a device for a drying section useful in the manufacture of paper or paperboard as a fibrous web (5). The drying section comprises a drying drum (1) which is rotatable about a first axis (2) of rotation and is arranged such that the fibrous web is brought to run in contact with the surface of the drying drum, at least about a part of its circumference, and to be dried thereby. The device includes a press roller (7, 9), which is rotatable about a second axis (8) of rotation which is essentially parallel with the first (2) axis of rotation, and arranged to, with a line load, bear against the fibrous web (5), whereby the drying drum (1) constitutes a support to the press roller (7, 9), At least one press roller (7) is arranged at a corresponding longitudinal edge (6) of the fibrous web (5).
The invention also provides a method for manufacturing a fibrous web of paper or paperboard in which the fibrous web is dried in a drying section and in which a line load is applied against the fibrous web. The line load is only applied at the longitudinal edges of the fibrous web.
According to the invention, it has thus been discovered that a relatively simple action, in the form of the application of a line load at the longitudinal edges of the fibrous web, in the drying section, will give a surprisingly good effect on the longitudinal edges of the fibrous web, as a decreased surface coarseness and thickness.
The device according to the invention is realised by means of a line load-giving, rotatable press roller, which is arranged to have a rotatable drying drum as a support for the line load, which Press roller may have the same extension in CD as the drying drum, or at least as the fibrous web, or may have an extension only in the area of the longitudinal edges of the fibrous web, in which case at least one press roller is arranged at the respective longitudinal edges. The press roller/rollers has/have an axis of rotation which is essentially parallel with the axis of rotation of the drying drum. The press roller and the drying drum are rotating in opposite directions to each other. The press rollers may, at their outermost short ends, overlap the longitudinal edges of the fibrous web, or, they may end in phase with the longitudinal edges of the fibrous web.
The concept of xe2x80x9cessentially parallelxe2x80x9d will here comprise a small angle deviation, the object of which may be to achieve a line load which varies along the length of the press roller/rollers.
The device is, according to one aspect of the invention, arranged to operate at a dry substance of about 50-95%, preferably about 60-90% and even more preferred about 70-90%, in the fibrous web. Thereby, the press rollers may be arranged in connection with one drying drum only, or in connection with a number of drying drums.
According to another aspect of the invention, the press roller/rollers is/are arranged in a position which corresponds to the inlet of the fibrous web to, or the outlet from, the drying drum, in which position the fibrous web runs freely, i.e. without a drying wire.